Chlorine, hydrogen and aqueous alkali metal hydroxide may be produced electrolytically in a diaphragm cell wherein alkali metal chloride brine, e.g., sodium or potassium chloride brine, is fed to the anolyte compartment of the cell, chlorine being evolved at the anode, the electrolyte percolating through a liquid permeable diaphragm into the catholyte compartment wherein hydroxyl ions and hydrogen are evolved at the cathode.
The diaphragm which separates the anolyte compartment from the catholyte compartment must be sufficiently porous to permit hydrodynamic flow of brine but must also inhibit back migration of hydroxyl ions from the catholyte compartment into the anolyte compartment as well as prevent mixing of evolved hydrogen and chlorine gases which could pose an explosive hazard.
Asbestos or asbestos in combination with various polymeric resins, particularly fluorocarbon resins (so-called modified asbestos) have long been used as diaphragm materials. Recently, due primarily to the health hazards posed by asbestos, numerous non-asbestos or synthetic diaphragms have been developed and are extensively described in the art. Such synthetic diaphragms are typically made of fibrous polymeric material resistant to the corrosive atmosphere of the cell and are typically made using perfluorinated polymeric material, e.g., polytetrafluoroethylene (PTFE). Such diaphragms may also contain various other modifiers and additives, e.g., inorganic fillers, pore formers, wetting agents, ion exchange resins or the like. Some of said synthetic diaphragms are described, for example, in U.S. Pat. Nos. 4,036,729; 4,126,536; 4,170,537; 4,210,515; 4,606,805; 4,680,101; 4,853,101 and 4,720,334.
Regardless of the nature of the diaphragm, i.e., be it asbestos, modified asbestos or synthetic, variations are often observed in cell operating characteristics, e.g., variations in diaphragm permeability and porosity, cell voltage, current efficiency and excessive hydrogen content in the evolved chlorine.